The invention relates to the mass-selective radial or axial ejection of stored ions from linear ion traps.
Linear quadrupole ion traps operate with an essentially quadrupolar radio-frequency field between four pole rods. The arrangement has been known since Wolfgang Paul; the basic principle is described in the same patent as the so-called xe2x80x9cthree-dimensional quadrupole ion trapsxe2x80x9d with ring and end caps (W. Paul and H. Steinwedel, DE 944 900; corresponding to U.S. Pat. No. 2,939,952). The basic arrangement which is often operated as a mass filter becomes a xe2x80x9clinear ion trapxe2x80x9d when rejecting fields are applied to the ends of the rod system, these being either DC voltage fields at the diaphragms or pseudo-potential fields as they appear in non-homogeneous radio-frequency fields. For example, pseudo-potential fields can be created by subsequent four-pole-rod systems Which are operated under different radio-frequency conditions.
(Comment on terminology: The term xe2x80x9clinear ion trapxe2x80x9d used here for rod systems has two meanings because a three-dimensional ion trap made up of ring and end-cap electrodes with an ideal quadrupole field is also termed xe2x80x9clinearxe2x80x9d. In an ideal three-dimensional quadrupole field, the radio-frequency field strength increases linearly both radially and axially, and the repulsing pseudo-forces also increase linearly. This produces a harmonic oscillator. In contrast, traps with superimposed hexapole and octopole fields do not display a linear increase in the fields and are therefore also called xe2x80x9cnonlinear ion trapsxe2x80x9d. They form a non-harmonic oscillator and show the phenomenon called xe2x80x9cnonlinear resonancesxe2x80x9d. The ion traps made up of four pole rods, which are referred to here as a xe2x80x9clinear ion trapxe2x80x9d, are sometimes called xe2x80x9ctwo-dimensional trapsxe2x80x9d because the fields only change along two coordinates (x, y) and remain constant along the third coordinate (z). This explains the term xe2x80x9cthree-dimensional ion trapxe2x80x9d for the trap with ring and endcaps, where the fields change in all three spatial coordinates. From the point of view of terminology, it would be better to make a distinction between xe2x80x9crod-system ion trapxe2x80x9d and xe2x80x9cring-endcap system ion trapxe2x80x9d, but the term xe2x80x9clinear trapsxe2x80x9d is now widely in use in the literature.)
A linear ion trap, as defined here, is disclosed in U.S. Pat. No. 5,420,425 (M. B. Bier and J. E. Syka, corresponding to EP 0 684 628 A1), operating with mass-sequential, radial ion ejection after dipolar resonance excitement through a slit in one of the pole rods. The mass spectra are scanned using a detector attached to the outside (or even two detectors outside two slits in opposing pole rods). The system is filled by injecting the ions into the rod system along the axis. Practically all the ions injected can be captured and storedxe2x80x94whereas in the case of the three-dimensional ion traps, only a few percent of the ions injected can be captured and stored. The advantages of an ion trap such as this are, firstly, that the filling behavior of the system is more efficient and, secondly, the spatial charge has much less influence on the ejection behaviorxe2x80x94higher filling levels can be therefore used without any reduction in resolution by space charge. In comparison to three-dimensional ion traps, where in most cases several spectra have to be added in order to produce a high quality spectrum which can be evaluated well, with linear traps, one spectrum is sufficient. The disadvantages are that the parallel adjustment of the pole rods must be extremely accurate (which is not usually the case for three-dimensional ion traps), the electronics are very complex, and the scanning rate for mass spectra is not very high, which largely cancels out the advantage of only needing one scan spectrum.
An article, by J. W. Hager, xe2x80x9cA new linear ion trap mass spectrometerxe2x80x9d, Rapid Commun. Mass Spectrom. 2002, 16, 512-526 disclosed a system using axial, mass-selective ejection of ions from a linear ion trap. In this case, use is made of the fact that, in the fringing field of the linear quadrupole field in front of a diaphragm on the exit side, the ions are not only able to oscillate radially but also axially. The axial oscillations are produced between the repelling DC potential of the diaphragm and the repelling pseudo-potential of the non-homogeneous fringing field within a small, flat potential well. These axial oscillations are now coupled with the radial oscillations due to the non-homogeneous shape of the potential surfaces in the fringing field; in other words, the two oscillation systems exchange energy. The energy passes from one oscillator system into the other and then back again. For example, if ions are excited to oscillate in the radial direction, they oscillate briefly in the radial direction, then oscillate briefly in the axial direction and then in the radial direction again, and so on. If the potential barrier at the front due to the diaphragms is not high, then radially excited ions will be able, during the first oscillation in the axial direction, to overcome this potential barrier and can be measured by a detector at the output. The oscillation can be excited by applying a radio-frequency voltage at one of the diaphragms. This is an excitation in the form of quadrupolar excitation.
The advantage of this method is again the efficient filling behavior of the linear ion trap by injecting ions from the end. The ejection yield is quoted to be 20%. That is significantly lower than for three-dimensional ion traps but, particularly with an improved filling rate of almost 100% and a somewhat larger storage, it is more than compensated for. However, the quantity of ions collected must not be as large as with the device for radial ejection described above if the ejection process is not to be inhibited by the spatial charge and, in particular, if the mass resolution is not to be reduced.
Around 1960, Wolfgang Paul and his then colleague, Friedrich von Busch, had already discovered the phenomenon of nonlinear resonances with quadrupole filters. Later, this phenomenon was mainly studied on three-dimensional quadrupole ion traps.
In three-dimensional quadrupole traps, where a radio frequency voltage is applied between the ring electrode and the two end-cap electrodes, ions can oscillate in an axial direction between the end caps and also in a radial direction parallel to the plane of the ring. The oscillations are sinusoidal with a mass-specific frequency of xcfx89(m/z), and are called the fundamental oscillations or secular oscillations of the ions. On these slower sinusoidal oscillations are imposed the rapid oscillations xcexa9 of the driving frequency, which is electrically applied at the ring electrode. According to the laws of trigonometry for the multiplication of two sinusoidal functions of different frequency, the multiplicative superimposition of sinusoidal oscillations leads to side bands with the main components (xcexa9xe2x88x92xcfx89) and (xcexa9+xcfx89) which, in this case, will be called Mathieu side bands because they appear as the solutions to the Mathieu differential equations for ion movement in the ion traps. Weaker components are (2xcexa9xe2x88x92xcfx89) and (2xcexa9+xcfx89), in general (nxcexa9xe2x88x92xcfx89) and (nxcexa9+xcfx89).
If higher multipole fields are superimposed on the quadrupole field due to electrical or mechanical distortions, overtones of the fundamental oscillation are produced, as is generally known for distorted oscillators. If the distortions are symmetric, the overtones produced are 3xcfx89, 5xcfx89 and 7xcfx89 and so on, where xcfx89 is the fundamental oscillation of the ions. Asymmetric distortions produce all overtones 2xcfx89, 3xcfx89, 4xcfx89, 5xcfx89 and 6xcfx89 and so on. Asymmetric distortions are obtained by superimposing higher multipole fields with odd numbers of pole pairs (such as hexapole and decapole fields) and symmetric distortions are obtained by superimposing multipole fields with even numbers of pole pairs (such as octopole and dodecapole fields).
Nonlinear resonances appear if the frequencies of the overtones match the Mathieu side-band frequencies. The energetic side-band frequencies, which arise from the voltage of the driving radio frequency of the ion trap, pump energy into the ions by exciting their oscillation overtones, just as a bell is made to vibrate by exciting its overtones. The oscillation amplitudes of the ions increase until the ions are eliminated from the ion trap, either by passing through apertures or by colliding with the electrodes.
A quadrupole field is also formed between the four pole rods of the linear ion trap. This field, however, only changes in two dimensions, while remaining constant along the axis of the pole rods (if the fringing fields at the ends of the rod systems are disregarded). The direction along the axis is called the z direction and the two directions between the two pairs of opposing rods are called the x and y directions. In a linear ion trap, the ions can oscillate in the x direction, the y direction or in both directions at the same time.
In the linear trap, it is also possible to produce overtones of ion oscillation by superimposing higher-order multipole fields. Mathieu side bands occur here as well. Here too, nonlinear resonances can be set up, as already discovered by Paul and Busch.
Different from three-dimensional ion traps, where superpositions of higher fields can only be achieved by mechanical distortions, the fields in linear ion traps can be distorted by mechanical means and by electrical means.
Mechanically caused field distortions arise, for example, by changes in the distance of individual pole rods from the center, by the use of cylindrical pole rods of different thicknesses or by hyperbolic pole rods with asymptotes which are not at right angles to one another. In particular, it is possible to generate a mathematically known mixture by mathematically simulating the equipotential surfaces of a desired superimposition of multipole fields. By using these mechanical means, it is possible to superimpose both xe2x80x9cevenxe2x80x9d and xe2x80x9coddxe2x80x9d multipole fields.
Electric field distortions are produced by changing the amplitude of the radio-frequency voltage of one pole rod relative to the voltage of the opposite pole rod. In this way, it is possible to produce superimpositions with xe2x80x9coddxe2x80x9d multipole fields. In comparison to mechanical distortions, electrical distortions have the advantage that the strength of the superimposition with higher xe2x80x9coddxe2x80x9d multipoles can be adjusted electronically, while mechanical distortions are not so easy to readjust to contain other components of higher multipoles. If the voltage at two opposing pole rods is altered in the same sense, then there is no superimposition with higher multipolesxe2x80x94only the axial potential in relation to the outside is superimposed by a component of the radio-frequency voltage.
The invention creates nonlinear resonance conditions by wilfully introduced distortions and exploits these nonlinear resonances advantageously for ion ejection as well in radial as in axial direction. The ejection in radial direction can thereby be made unidirectional.
A distortion of the essentially quadrupolar radio-frequency fields can be achieved by superimposing weak higher multipole fields, especially including higher odd multipole fields; either by disadjusting or reshaping the rod system mechanically, while keeping the rods strongly parallel, in particular by changing the distance between the individual rods and the center, or electrically by offsetting the radio frequency voltage at a particular rod.
It is possible to set up xe2x80x9cevenxe2x80x9d multipole fields (such as octopole or dodecapole fields etc.) by symmetric distortion of the electric field in either the x or the y direction by mechanical means solely, and so-called xe2x80x9coddxe2x80x9d multipole fields (such as hexapole or decapole fields etc.) by asymmetric distortions, produced as well by mechanical or by electrical means.